1. Field of the Invention:
The present invention relates to an overload indication device for a lever hoist to prevent it from being operated under an overload condition.
It will be understood that the expression "lever hoist" used herein will means a manually operated hoist having a lever which is repeatedly rocked by a hand of an operator to rotatively drive a load sheave around which extends a chain or wire rope having at one end a loading hook.
2. Description of the Prior Art:
Various devices for preventing overloading of lever hoists have been suggested. There has been used a lever hoist including a main lever provided with a change-over ratchet for driving a change-over ratchet wheel and a handle pivotally secured to a free end of the main lever, between which handle and main lever a shear pin is provided adapted to be sheared when being subjected to an overload. With this hoist however, there is a risk in operation due to a rapid rotation of the handle relative to the main lever when the shear pin is sheared owing to the overload. The sheared pin must be replaced by a new one, which requires troublesome and time consuming operation. As the sheared pin can not be used again, it may increase the maintenance costs. Moreover, it is difficult to produce shear pins having uniform shearing strengths which would be determined by severe tolerance and uniform quality of material. Dimensional error and difference in quality of material cause differences in the shearing strength of the shear pins which would not achieve an overload prevention with a high accuracy.
Another lever hoist has been proposed which includes a main lever and a handle of which one corner of one end is pivoted to one corner of a free end of the main lever, and a pellet accommodated in engaging apertures formed in the other corners of the ends of the main lever and handle and urged by a compression spring. When the hoist is in an overloaded condition, the pellet moves from the engaging apertures against the compressive force of the spring with the aid of camming action of a rounded end of the pellet, as disclosed in Japanese Utility Model Application Publication No. 34,393/73. With this arrangement, there is a tendency for the pellet to be rapidly removed from the engaging apertures resulting in a rapid rotation of the handle relative to the main lever, which is dangerous in operation. Furthermore, the pellet moves from the apertures while frictionally sliding thereon, thereby not achieving an overload prevention with a high accuracy.
A further lever hoist has been suggested including a main lever and a handle of which one corner of one end is pivoted to a free end of the main lever, and a pressure receiving member urged by Belleville springs and in contact with the pivoted end of the handle through a slipper member. In use, the handle is pivotally moved relative to the main lever corresponding to a load acting upon the hoist thereby detecting a loaded condition by the angle of the handle, as disclosed in U.S. Pat. No. 3,921,471. With this arrangement, the overloading is determined by an operator by an observation of the angle of the handle, so that an exact determination of overloading is very difficult. Moreover, after the handle has been once pivotally moved relative to the main lever, if the handle is released, the handle is returned by the action of the Belleville springs to an original position aligned with the main lever, so that the determination of the overloading is difficult.